1. Field of the Invention
The present invention relates to the process of photolithography used to manufacture semiconductor devices. More particularly, the present invention relates to a photomask of an exposure apparatus of photolithographic equipment, and to a method of manufacturing the photomask.
2. Description of the Related Art
Photolithographic techniques have been widely used, in the manufacturing of semiconductor devices, to form patterns on a wafer by transcribing a pattern of a photomask onto a wafer. In these techniques, a photoresist layer is formed on the wafer. The photoresist layer is exposed to light directed through the photomask, whereby an image corresponding to the pattern of the photomask is formed on the photoresist layer. The exposed photoresist layer is then developed, which process patterns the photoresist layer. Then, a layer(s) underlying the photoresist layer is/are etched using the patterned photoresist layer as a mask to thereby pattern the underlying layer(s).
Moreover, the design rules for manufacturing these devices are constantly decreasing to meet the demand for semiconductor devices having higher integration densities. However, limits in the resolution of the exposure process of photolithography pose serious obstacles to manufacturing semiconductor devices having highly integrated circuit patterns. In particular, the uniformity that can be attained in the dimensions of the pattern formed on the wafer is limited by the resolution that can be attained in current photolithographic exposure techniques.
Furthermore, the same photomask is used in photolithography to form a pattern whose elements all have the same (critical) dimension. Nonetheless, the dimensions of these elements of the pattern may vary considerably depending on the place on the wafer where each of the elements is formed. This is because the uniformity of the dimensions of patterns formed on a wafer using a common mask is affected by various factors including the processes by which the photoresist layer is formed, e.g. coating and baking processes, the characteristics of the exposure apparatus including those of the photomask, the process by which the photoresist layer is developed, and the process by which layer(s) underlying the patterned photoresist layer is/are etched. The shot uniformity or in-field uniformity, namely, the differences between corresponding dimensions within the pattern formed on the wafer, is especially dependent upon the characteristics of the exposure apparatus and photomask.
Thus, various ways to increase the resolution of the exposure process have long been a matter of concern in the field. For example, the use of a diffraction grating or a filter to vary the illumination produced by the light source throughout the course of the exposure process has gained in popularity among photolithography techniques aimed at increasing the resolution of the process. However, this technique has not been found to be entirely successful in increasing the uniformity of the dimensions of patterns on a wafer. Also, efforts at reducing a difference in the critical dimensions of each of the constituent elements of a pattern on a wafer have been undertaken by providing the photomask with a filter, using a laser pulse as exposure light, and varying the energy of the laser pulse. However, such efforts are far from being put to practical use because of the difficulty in freely varying the intensity of the laser pulses to the extent required to attain uniformity in the critical dimensions.